Immune defense against HIV is usually ineffective. Viral escape due to error prone RNA-dependent DNA polymerase mediated replication is a constant source of variants that can evade existing defenses. Despite this antigenic variability, a small fraction of HIV-infected individuals make broadly neutralizing antibodies, and are protected from disease. Recently, we and others have defined the structures of epitopes recognized by some of the broadly neutralizing antibodies. Despite our understanding of the structural nature of these epitopes, this knowledge has not translated into effective vaccines that elicit broadly neutralizing antibodies. We hypothesize that inefficient epitope display and insufficient adjuvancy are major hurdles to overcome in HIV vaccine development. To address this hypothesis we propose to engineer flagellin molecules that will display HIV epitopes and posses potent adjuvant activity, and thus transform bacterial flagellin into an HIV vaccine. Flagellin possesses unique characteristics that promote its antigenicity. Flagellin is exposed on the cell surface as a polymer (the flagellum), and flagellin is recognized by the innate immune system through multiple pathways, including TLR5 and caspase-1. We hypothesize that HIV epitopes that are appropriately incorporated into bacterial flagellin will be potent immunogens and elicit broadly neutralizing antibody responses. To test this hypothesis, we propose to perform a detailed analysis of the structural components of flagellin that are required for antibody responses, and to define strategies to incorporate antigenically intact HIV epitopes into bacterial flagellin. The three specific aims of this grant application are: 1) define components of flagellin and innate immune pathways required for potent immune responses, 2) design HIV epitope-flagellin fusions that retain innate immune stimulatory capacity and bind broadly cross-reacting antibodies, and 3) analyze immunogenicity of HIV epitope-flagellin fusions. Immune defense against human immunodeficiency virus (HIV) is usually ineffective due to tremendous variability. However, a small fraction of HIV-infected individuals make protective antibodies to conserved structures, and these antibodies recognize the majority of the HIV strains. We propose to engineer bacterial flagellin molecules that will display conserved HIV epitopes for use as potent immunogens that elicit broadly neutralizing antibody responses to HIV.